The present invention relates to ammonium nitrate and more particularly to a crystalline growth inhibitor for ammonium nitrate.
Ammonium nitrate (AN) is used in solid propellant formulations for gas generators, rocket motors and in explosives. One major disadvantage with AN is that it is well recognized as being inherently dimensionally unstable when heated or cooled. Pure and dry ammonium nitrate (AN) exists in at least five different polymorphic forms at atmospheric pressure. When heated, AN experiences about a 3% volumetric contraction at -18.degree. to 0.degree. C. and about a 3.5% volumetric expansion between 32.degree. to 50.degree. C. Upon cooling, the reverse occurs but at different temperatures with a resultant net volumetric change after thermal cycling.
Considerable hysteresis occurs within any of these transitions. For example, the phase II to III transition occurs anywhere between 46.degree. C. and 84.degree. C. The phase III to II transition occurs between 84.degree. C. and 90.degree. C. The phase IV to III transition occurs with considerable hysteresis, between 32.degree. C. and 55.degree. C. and III to IV between -1.degree. C. to 35.degree. C.
Unfortunately, this expansion and contraction occurs in the temperature range of use in rocket motors and explosives, in particular, in the range of --55.degree. C. to 80.degree. C. There have been catastrophic failures of rocket motors, as a result of the excessive pressures caused by these volumetric changes. A straight forward solution to prevent such failures is therefore of great interest to propellant designers.
One solution to this dimensional instability problem is to introduce 10% to 17% by weight of potassium nitrate to the AN. However, this approach results in large amounts of undesirable residues in the combustion products. These residues tend to plug and corrode the rocket nozzle.
Another solution is described in U.S. Pat. No. 4,552,736. This patent describes the addition of 0.5% to 2.0% by weight of potassium fluoride (KF) as a specific crystal growth inhibitor to the molten phase of AN. However, our experiments have shown that this percentage range of KF does not consistently produce the desired result of true phase stabilization. The hot melt process as described in this patent is also very hazardous to perform. The hot melt process requires heating anhydrous AN to above its melting point of 167.degree. C., adding and mixing anhydrous KF with the AN, and then spray drying the stabilized AN in a chilled condition. This process is inherently susceptible to production of unwanted and uncontrolled AN combustion.
In our experiments, there were indications that propellant formulations having AN with 2% KF did not become stabilized when prepared by using either the hot melt method described in U.S. Pat. No. 4,552,736 or an aqueous method of preparation. In these experiments, separate batches of an OMAX 451 gas generator propellant formulation were prepared. OMAX 451 is a proprietary propellant formulation which is available from Olin Corporation. This formulation is a propellant mix of AN with cis-polybutadiene polymer as a binder. The AN used in this formulation, which included 2.0% KF, was made by both the hot melt method and the aqueous method. The resultant OMAX 451 propellant formulation crystals were formed into grains which were then subjected to hot and cold thermal cycling to determine the amount of volumetric growth. The results are given below in Table 1.
TABLE 1 ______________________________________ THERMAL CYCLING DATA OF "OMAX" 451 (CIS POLYBUTADIENE) PROPELLANT % VOLUMETRIC GROWTH AN with 2% KF AN with 2% KF (hot melt method) (aqueous method) ______________________________________ 20 -3.45 2.5 30 4.8 5 40 8.3 9 (45)-crack) 60 11 &gt;20% crack ______________________________________
As can readily be seen, the AN with 2% KF made by the hot melt method of U.S. Pat. No. 4,552,736 as well as the AN with 2% KF made by the aqueous method did not produce formulations which exhibit stability. In both cases the thermal cycling resulted in greater than 3% volumetric growth within 30 cycles. In addition, the OMAX 451 formulation grain made by the aqueous method even cracked severely after 45 cycles, a highly undesirable result. Accordingly a safer and more controllable production method remained elusive and a consistently reproducible phase stabilized AN product was still needed.